where the variable is , and the problem data
are , and
.
Another version of l1_ls handles the same problem, with the
additional constraint that is nonnegative.

l1_ls is developed for large problems.
It can solve large sparse problems with a million variables with
high accuracy in a few tens of minutes on a PC.
It can also efficiently solve very large dense problems, that
arise in sparse signal
recovery with orthogonal transforms, by exploiting fast algorithms
for these transforms.